1. Field of the Invention
The invention relates in general to a fabricating process of an integrated circuit, and more particularly, to a method of mending the erosion of a bonding pad caused by a residue on the bonding pad.
2. Description of the Related Art
When the main structure of an integrated circuit is formed, a protection layer is formed on the surface of the integrated circuit to protect the underlying devices or components of the integrated circuit. An ideal protection layer is formed with characteristics of uniform deposition, anti-cracking property, nonexistence of voids, resistance of penetration of moisture and alkaline ions, hardness, and strength of sustaining mechanical damage.
Silicon nitride and phospho-silicate glass (PSG) are commonly used for forming the protection layer. Silicon nitride which has a density higher than PSG is superior to resist the penetration of moisture and alkaline ions and to protect the integrated circuit from being damaged by an external mechanism. On the other hand, PSG contains phosphorous which has the gettering property, so that the moisture and alkaline ions can also be absorbed effectively to prolong the lifetime of the integrated circuit. Typically, a polyimide layer is formed after the formation of a silicon nitride layer for waterproofing.
The polyimide layer can be formed by a method of using one photo-mask or two photo-masks. The method of using one photo-mask is more straightforward and less time consuming than the method of using two photo-masks.
Aluminum is one of the most popular conductive materials applied in very large-scale integration (VLSI) currently. The aluminum has a good conductivity and adhesion, moreover, the cost of aluminum is low, and it is easily deposited and etched. Therefore, the bonding pad of an integrated circuit is often formed by aluminum.
In a conventional method, after the formation of a bonding pad on a substrate, a PSG layer is formed to cover the bonding pad and the substrate, and a silicon layer thicker than the PSG layer is formed on the PSG layer. For the purpose of protection, a polyimide layer is further formed on the silicon nitride layer. The polyimide layer can also function as a photo-resist layer for a photolithography process to define an opening penetrating through the silicon nitride layer and the PSG layer to expose the bonding pad. Since the polyimide can be removed in organic solution, after the etching process, a thorough cleaning process can not be performed by organic solution properly.
During the process of etching the silicon nitride layer and the PSG layer, an etchant containing fluorine is typically used. However, the fluorine cannot be properly cleaned away leaving residue on the surface of the bonding pad. With the residue of fluorine, the surface of the bonding pad is easily eroded affecting the profile and yield in the subsequent bonding process. Moreover, wafers being eroded sometimes fail the required quality and must be abandoned, causing waste.